docs: def posr(string, substr, start):
      """
      Find the first occurrence in a string of a substring, starting at
      a specified location, searching backward.
  
      http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/posr_c.html
  
      :param string: Any character string.
      :type string: str
      :param substr: Substring to locate in the character string.
      :type substr: str
      :param start: Position to begin looking for substr in string.
      :type start: int
      :return:
              The index of the last occurrence of substr
              in string at or preceding index start.
      :rtype: int
      """
      string = stypes.stringToCharP(string)
      substr = stypes.stringToCharP(substr)
      start = ctypes.c_int(start)
      return libspice.posr_c(string, substr, start)